The invention relates to a data transmission system comprising a plurality of reconfiguring devices, each device having a plurality of input and output data links, the devices being connected in a ring whereby for each reconfiguring device data is received on one data input link and transferred to one data output link such that data may be passed between all the reconfiguring devices along the ring. The invention also relates to a method of operating a reconfiguring device forming part of such a data transmission system.
The invention is particularly applicable to local area networks. Local area networks are described in an article entitled "An Introduction to Local Area Networks" by David Clark, Kenneth Pogran, and David Reed in Proceedings of the IEEE, Volume 66, No. 11, pages 1497-1517. These data transmission systems enable a number of data processing devices, for example microprocessors, to communicate with each other at high speeds over distances typically in the range 10 to 10,000 meters. The system may include both the reconfiguring devices mentioned above which will have a plurality of input and output data links and one or more non-reconfiguring devices hereinafter referred to as "single link pair" devices which have a single data input link and a single data output link. Each of the devices may provide an interface between the associated data processing device or devices and the ring or provide a network function. It should be understood that the data links referred to allow the devices to communicate with a ring. Additional links are provided to connect each device to one or more host devices.
One advantage of a ring over other topologies is that the protocols which organise the transmission of data around the ring can be a fairly simple nature. By comparison with packet switching techniques these have the advantage that as data arrives at each device it is not necessary to decide to where it must be routed since it is simply passed from the one data input link to the one data output link. The only decision, during normal use, that a device has to make is whether the data is addressed to it in which case it must be read.
One of the problems with a ring topology arises on the occurrence of a fault since in theory this could cause complete break down of the transmission system. Some prior proposals have been made to deal with this problem.
No. GB-A-1,570,923 discloses a transmission system arranged in a ring topology in which there are additional connections between some of the stations in the ring so that if a fault should occur it can easily be bypassed using the second connection. The problem with this arrangement is that these special stations need to determine the destination of data since there will be a choice of paths for the data to take and a number of the other stations will be bypassed.
No. EP-A-0074672 and No. GB-A-2114858 disclose so-called dual ring arrangements in which a number of stations are connected together in series by a pair of communication channels which enable data to be transmitted in opposite directions between the stations. Data is normally transmitted in one direction around the ring along a set of primary channels. If a fault occurs then any one of the stations can cause data to transfer from a primary channel to an auxiliary channel so that a sub-ring can be built up allowing the fault to be bypassed. Although these arrangements allow the integrity of the ring to be at least partially maintained despite the development of a fault they do not provide a simple method for allowing the original ring to be reconfigured once the fault has be cured.